1. Field of the Invention
The present invention relates to an image encoding device.
2. Background of the Related Art
H. 263, and MPEG4, methods for compression encoding of an image signal, encode motion vectors based on DPCM, i.e., encode a difference of a predicted value on the basis of pixel values that can be known at a decoder and the current pixel value through quantization procedure and VLC (Variable Length Coding) procedure. Accordingly, the image compression encoding on the basis of DPCM are divided into a part for obtaining the predicted value, a part for obtaining the difference of the predicted value and the current pixel value to be encoded, a part for quantizing the difference, and a part for variable length coding a quantized difference. Upon the image compression encoding on the basis of DPCM, the compression performance depends on how well the predicted value is obtained.
Conventional prediction methods are classified into a linear prediction method and a non-linear prediction method. The former method is in general a technique describing how different weighted values are given to linear combinations of adjacent pixels. A DPCM encoder based on the linear prediction method is quite effective in case of flat areas even if it apparently has a restricted efficiency due to the irregularity of signal. When it is assumed that information ‘Z’ is given, in the latter case, it is theoretically verified that a maximum prediction value to information ‘Y’ in view of minimum mean square is a conditional expectation value E(Y|Z) [R. Gray and A. Gersho, Vector quantization and signal decomposition, Kluwer Academic Press, 1992.]. However, since such a theoretical prediction method cannot be embodied practically, a non-linear prediction value cannot be actually obtained on the basis of the theory.
The conventional linear prediction method and the non-linear prediction method are based on characteristics, and proximity of adjacent pixels. However, it frequently appears that usage of proximity of adjacent pixels is appropriate on actual images starting from cases such as boundaries or lines. Therefore, the image compression encoding cannot be solved with the usage of proximity.